Great mobile internet - but WiFi...

What can I do when our router have great connection with mobile internet, but Wi-Fi connection from my PC to this router working very bad? Signal quality is 76% - 86%, but site browsing is very lagging etc. (Connection sometimes fail, but bigger problem is low browsing (of websites) quality)... My brothers with Windows not have this problem.

Simple situation: I had to wait an half hour - until then it was impossible to reload this page. In previous operating system WiFi failed once per 6-12 hours. But site browsing worked - little bad, but it worked... Now I cannot browsing pages without spending lot of hours. I must waiting, waiting and waiting for loading pages... (Funny story - Aptitude working very nice... I can download packages from APT repository...)